1. Field of the Invention
This invention relates to an evaporative emission control system for an internal combustion engine for purging evaporative fuel generated from a fuel tank into an intake passage of the engine, which is capable of detecting an abnormality thereof.
2. Prior Art
Conventionally, an evaporative emission control system for an internal combustion engine has been widely used, which comprises a fuel tank, a canister for adsorbing evaporative fuel generated from the fuel tank, a purge control valve arranged in a passage communicating between the canister and the intake passage of the engine for control of purging of evaporative fuel absorbed in the canister into the intake passage. Further, a method of detecting an abnormality in this type of evaporative emission control system has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-102360 (hereinafter referred to as "Publication 1"), Japanese Provisional Patent Publication (Kokai) No. 4-362264 (hereinafter referred to as "Publication 2"), and International Publication No. WO 91/12426 (hereinafter referred to as "Publication 3").
Publication 1 discloses a method of determining that the evaporative emission control system is abnormal (the passage communicating between the fuel tank and the canister is clogged up), when the tank internal pressure detected by pressure-detecting means arranged in a pressure-detecting conduit of the fuel tank is higher than a predetermined value.
Publication 2 discloses a method of negatively pressurizing the evaporative emission control system by the use of negative pressure created with the intake passage of the engine, then isolating the interior of the evaporative emission control system to detect a change of pressure within the system over a predetermined time period, and determining that the evaporative emission control system is abnormal (there is a leak of evaporative fuel) when the change of pressure within the system is larger than a predetermined value.
Publication 3 discloses a method of providing pressure-detecting means for detecting pressure within the system, and a control valve for opening and closing a passage for introducing the air into the canister, and closing the control valve while at the same time opening the purge control valve to determine that the system is normal if the system can be negatively pressurized.
However, the above methods suffer from the following problems:
(1) The method of Publication 1 cannot detect a leak of evaporative fuel, since it determines that the system is abnormal simply when the tank internal pressure is higher than the predetermined reference value. PA1 (2) Although the methods of Publications 1 and 2 can detect a leak of evaporative fuel, it takes a fairly long time period to negatively pressurize the system to a predetermined level, and further, evaporative fuel is purged from the canister and the fuel tank into the intake passage of the engine during negative pressurization of the system for abnormality detection, which can make an air-fuel mixture supplied to the engine overrich over the long time period, causing degraded drivability and exhaust emission characteristics of the engine. PA1 diagnosis means for determining that the evaporative emission control system is normal when a value of the pressure within the part of the evaporative emission control system detected by the pressure-detecting means is lower than a predetermined reference value below atmospheric pressure.